Pneumococcal disease continues to be a leading cause of sickness and death in the United States and throughout the world. Each year, millions of cases of pneumonia, meningitis, bacteremia, and otitis media are attributed to infection with the pathogen Streptococcus pneumoniae. S. pneumoniae is a Gram-positive encapsulated coccus that colonizes the nasopharynx in about 5-10% of healthy adults and 20-40% of healthy children. Normal colonization becomes infectious when S. pneumoniae is carried into the Eustachian tubes, nasal sinuses, lungs, bloodstream, meninges, joint spaces, bones and peritoneal cavity. S. pneumoniae has several virulence factors that enable the organism to evade the immune system. Examples include a polysaccharide capsule that prevents phagocytosis by host immune cells, proteases that inhibit complement-mediated opsonization, and proteins that cause lysis of host cells. In the polysaccharide capsule, the presence of complex polysaccharides forms the basis for dividing pneumococci into different serotypes. To date, 93 serotypes of S. pneumoniae have been identified.
Various pharmaceutical compositions have been used to harness an immune response against infection by S. pneumoniae. A polyvalent pneumococcal vaccine, PPV-23, was developed for preventing pneumonia and other invasive diseases due to S. pneumoniae in the adult and aging populations. The vaccine contains capsular polysaccharides (CPs) from 23 serotypes of S. pneumoniae. As T cell-independent antigens, these CPs induce only short-lived antibody responses, necessitating repeated doses, which increases the risk of immunological tolerance. The antibodies raised against S. pneumoniae, termed anticapsular antibodies, are recognized as protective in adult and immunocompetent individuals. However, children under 2 years of age and immunocompromised individuals, including the elderly, do not respond well to T cell-independent antigens and, therefore, are not afforded optimal protection by PPV-23. Another S. pneumoniae vaccine, Prevnar, includes bacterial polysaccharides from 7 S. pneumoniae strains conjugated to the diphtheria toxoid protein. This vaccine induces both B and T cell responses. However, because it only protects against 7 pneumococcal serotypes, serotype replacement can render Prevnar ineffective. Serotype replacement has already been demonstrated in several clinical trials and epidemiologic studies, necessitating development of different formulations of these vaccines. An example is the recently introduced Prevnar 13, directed to 13 pneumococcal serotypes. Furthermore, the two Prevnar formulations are expensive to manufacture, greatly limiting their availability in the developing world. PPV-23, which consists of 23 purified but unconjugated polysaccharides, has broader coverage, but does not provide protection to children under the age of 2 years, a population which is at the highest risk for pneumococcal disease.
Thus, there remains a need to design more effective pharmaceutical compositions than the current strategies offer. In particular, such compositions need to incorporate novel or specific antigens that elicit an immune response against S. pneumoniae. 